Yeast cells carrying a mutation in the OXA1 nuclear gene are respiratory deficient and lack cytochrome oxidase activity. We successively examined the different steps in the expression of the mitochondrial genes encoding the cytochrome oxidase subunits and apocytochrome b in strains carrying the oxa1-79 mutation. The ox1-79 strains exhibit a total absence of cytochrome aa3 and a decrease in cytochrome b, even in a strain devoid of mitochondrial introns, in which cox1 and cytb mRNAs normally accumulate. The three mitochondrial-encoded subunits of cytochrome oxidase are still detectable although their amount is reduced, and apocytochrome b is synthesized normally. These results suggest that the OXA1 gene is primary required at a post-translational step in cytochrome oxidase biogenesis, probably at the level of assembly, although the oxa1-79 mutation leads to some pleiotropic secondary defects in earlier steps of mitochondrial gene expression. The OXA1 gene has been cloned, sequenced, and disrupted. The phenotypes of the oxa1::LEU2 and oxa1-79 alleles are similar. Interestingly, the OXA1 gene, located on the yeast chromosome VIII, is adjacent to the gene PET 122, which controls the initiation of cox3 mRNA translation. In addition, the predicted OXA1 protein is homologous to several putative prokaryotic and eukaryotic proteins, suggesting that the function of the OXA1 protein is important for respiration in all living cells.